The invention relates to a disc brake having a brake caliper, which fits over a brake disc and which has a caliper head and a caliper back connected thereto by tension struts. Brake pads are arranged in the caliper, which can be pressed against the brake disc from both sides and are secured radially by a retaining bracket extending in an axial direction of the brake disc. A measuring device is connected to an evaluation unit for determining the application force.
For many control tasks associated with preferably pneumatically or electromechanically actuable disc brakes, a precise knowledge of the application force applied during braking is important, this then serving as a measured variable.
Whereas it is possible with hydraulically actuated disc brakes to produce a suitable signal that correlates with the application force via the pressure of the brake fluid, determining the application force in electromechanically operated disc brakes is possible only with a considerable outlay in terms of design and production engineering.
To achieve stable closed-loop control behavior, such disc brakes are operated under position control. In this case, the position of the brake actuating element is determined by detecting the angular position of the drive motor, with the relationship between the angular position and the effect achieved by the disc brakes being subject to significant external influences. These include, for example, the wear condition, a temperature-dependent compressibility of the brake pads, the size of the release clearance, i.e. the spacing between the brake pad and a brake disc in the unbraked position, and the effects of oblique wear of the brake pads.
For these reasons, it is advantageous to obtain accurate feedback on the braking effect achieved by detecting appropriate physical states of the disc brake.
One known method of achieving this is to detect the application force by means of force sensors arranged in the force transmission path within the brake mechanism (DE 101 48 472 A1). However, it has been found in practice that these force sensors, which have to absorb clamping forces of more than 100 kN in commercial vehicle brakes, are relatively fault-prone, especially due to the rough operating conditions that prevail.
In addition to the resulting short life of the sensors, there is also the fact that the replacement which is then required can be carried out only with a considerable and, to that extent, regrettable amount of installation work since the positioning of the sensors within the brake mechanism, as necessitated by their operation, makes replacement very difficult.
Moreover, in the case of a disc brake with a plurality of pressure pieces or actuating spindles by which the brake pads can be pressed against the brake disc, there is a need for a corresponding number of sensors, and this naturally gives rise to considerable costs both for the procurement of the sensors and for their installation and connection as well as for an associated electronic evaluation system.
Overall, the method used hitherto for determining the application force, and the disc brake configured for this purpose, do not provide a satisfactory solution.
EP 1 748 213 A1 discloses a disc brake on which the application force is determined by a device, being determined substantially from the degree of expansion of a gap provided in the brake caliper. However, this presupposes an appropriate configuration of the brake caliper, which incorporates an L-shaped gap. Overall, this design of the brake caliper is associated with considerable disadvantages, among which are not only the relatively involved production process but also the permanent lack of functional reliability, which is limited especially by the risk of contamination of the gap formed.
It is the underlying object of the invention to develop a disc brake of the type in question in such a way that the application force can be determined with a low outlay in terms of design and production engineering and that the operating costs of the disc brake are reduced.
This object is achieved by a disc brake having a brake caliper, which fits over a brake disc and which has a caliper head and a caliper back connected thereto by tension struts. Brake pads are arranged in the caliper, which can be pressed against the brake disc from both sides and are secured radially by a retaining bracket extending in an axial direction of the brake disc. A measuring device is connected to an evaluation unit for determining the application force. The measuring device has two basic elements, which can be moved relative to one another in the direction of brake application, one of which is designed as a sensor and the other as a corresponding part operatively connected thereto, one basic element being arranged on the retaining bracket, which is clamped at one end on the caliper head or on the caliper back, and the other basic element being arranged on the brake caliper at the opposite end from the end at which the retaining bracket is clamped.
The invention exploits the fact that an application-induced spreading of the brake caliper, which is known from analyses of the deformation behavior of brake calipers for pneumatically actuated disc brakes, is proportional to the application force applied. At the same time, this relationship is stable over the life of the brake caliper and is subject essentially to only minimal disturbing influences, even in different operating states.
The invention is distinguished especially by the fact that it is extremely simple to implement and that the structural embodiment required for this purpose basically manages with a small number of parts that are simple to produce and to install, and which are furthermore subject to only very small operational stresses. The low outlay in terms of design and production engineering results especially from the use of the retaining bracket customarily used on disc brakes for commercial vehicles as a corresponding part to the sensor.
In one variant embodiment, the motion of the caliper back relative to the caliper head, counter to the direction of brake application, is determined by a change in the position of the retaining bracket, which is rigidly attached to the caliper head or to the caliper back, relative to the oppositely situated sensor attached to the brake caliper, and transmitted to an evaluation unit.
In principle, it is sufficient to install just one sensor, which can be attached to the outside of the brake caliper and arranged in such a way as to be unaffected by the forces acting on an application device. This results in extremely favorable positioning for installation, electrical connection and service purposes, resulting in low servicing and maintenance costs.
Any temperature dependence of the measurement signal can be adequately reduced by suitable measures. The change in the spacing between the caliper back and the caliper head resulting from heating of the brake caliper due to braking and the change in the variable which thus has to be measured is compensated by ensuring that the unilaterally fixed component corresponding to the sensor is subject to the same thermal expansion as the brake caliper itself or as the tension struts of the latter through an appropriate design configuration, an appropriate choice of materials, and an appropriate arrangement on the brake caliper.
Compensation of any faults which may occur owing to differences in thermal expansion can be accomplished through automatic initialization of the evaluation system at predetermined time intervals between the measurement operations and, if required, a correction calculation based on an evaluation of the energy converted by the brake. This initialization can be accomplished by providing for the spacing signal between the fixed component and the sensor to be detected at one-minute intervals, for example, in the unactuated position of the brake.
In the event of a braking operation, the difference between the current measurement signal measured during braking and the last value measured in the unbraked position, a zero point, is formed. In determining the application force, it is thus not the absolute value of the spacing signal which is taken as a basis but the change in said signal with respect to the respective quantity determined before a braking operation.
Further possibilities are opened up through comparison of the measured value for the caliper spread with a position signal from the motor in the case of electromechanical brake application. The determination of the application travel of the brake via the motor position signal incorporates not only the caliper spread but also the release clearance of the brake and a compression stroke due to compressibility and unevenness of the brake pads.
Given knowledge of the extent of caliper spread and brake pad wear, obtained, for example, by means of a brake pad wear sensor that is customary in production brakes, it is possible to draw conclusions about the state of the brake pads and of the current release clearance.
Given continuous monitoring of the correlation between the motor position signal and the caliper spread determined, emergency operation of the brake using the motor position signal is furthermore possible with little loss of accuracy if the sensor for measuring the caliper spread fails.
According to a preferred embodiment of the invention, the sensor can be designed as a Hall-effect sensor in correspondence with the retaining bracket forming the basic part, or with a separate basic element attached to said bracket, with the Hall-effect sensor, which is fixed with respect to the retaining bracket, being energized, while the retaining bracket is designed as a magnet or carries a magnet that moves past the Hall-effect sensor. This method makes possible a correspondingly large linear measurement range compared with one in which the retaining bracket is moved toward the sensor.
As an alternative, the corresponding part can be attached to the brake caliper and the sensor attached to the retaining bracket.
The use of a Hall-effect sensor means that only the relative movement of the retaining bracket is detected, the initial state before the start of a braking operation in each case being compared with the actual state that arises during the braking operation. Production-related tolerances or their effects on the result of measurement are thus excluded, while any changes which occur due to thermal expansion are negligible owing to the short duration of braking.
Given a maximum possible movement of the retaining bracket relative to the Hall-effect sensor of 1.5 mm, for example, a linear measurement range of 4.5 mm for the Hall-effect sensor is used. That is to say scatter in the region of 3 mm in the absolute value, due to the effects of tolerances or thermal expansion for example, i.e. twice the maximum possible measured value, can be compensated.
Since this means that it is no longer the absolute value of a spacing which is detected but now only the change in the position of the retaining bracket relative to the Hall-effect sensor, no compensation measures for disturbing influences are required. Likewise, only slight demands, if any, are made on the accuracy and, especially, the repeatability of the sensor device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawing.